1. Field of the Invention
The present invention generally relates to hearing aids, and more specifically to a hearing aid comprising a digital signal processor and a standard hearing aid battery for the supply of power to the digital signal processor.
2. The Prior Art
WO-A-91/08654 describes a hearing aid comprising a microphone, an output transducer, a digital signal processor interconnected between the microphone and the output transducer, and a power source including a standard hearing aid battery for the supply of operation voltage for said digital signal processor.
In its fresh condition a normal hearing aid battery supplies a voltage of about 1.3 V. During its active life the battery can supply current sufficient for the operation of the hearing aid while the voltage gradually declines down to a voltage of about 1 V. From this instant onwards the power supplying capacity of the battery drops rapidly.
In prior art hearing aid technology it is well known, e.g. from EP-A-0 335 542, U.S. Pat. No. 4,539,440 and U.S. Pat. No. 5,581,455 to provide operation voltages higher than the nominal battery voltage for certain processing circuits or components, e.g. EEPROM memories and microphone circuits, by means of voltage step-up converters, mostly in the form of switched capacitor networks designed e.g. as so-called charge pump voltage multipliers.
Further examples of use of voltage regulators in hearing aids have been disclosed e.g. in DE-A-27 38 339, DE-C-31 34 888, DE-A-197 02 151 and WO-A-96/03848. Thus, DE-A-197 02 151 discloses a hearing aid comprising a voltage regulator capable of providing a number of stabilized supply voltages that may be higher or lower than the nominal battery voltage.
Outside the hearing aid field a voltage dropping circuit in MOSFET technology with reduced power consumption has been disclosed in U.S. Pat. No. 4,205,369.
Whereas in conventional hearing aid technology the major power supply requirement has been to provide stabilized operation voltages sufficiently high for the operation of signal processing circuits, while the provision of voltages below the nominal battery voltage has only been resorted to for voltage stabilization or provision of reference voltages, further lowering of the operation voltage has been considered inconvenient, since it would result in loss of processing speed. In certain parts of digital hearing aids such as a D/D output converter, which are responsible for a main part of the power consumption, adaptation for operating at lower voltages would only result in a current increase and would achieve no saving in power consumption for the same output power from the D/D converter.
Moreover, in small size hearing aids with a low voltage drop and a current drain of a few mA or even a fraction of mA only, the potential power saving by reduction of the operation voltage has been considered outside interest due to the complex circuitry required to implement a low loss, stabilized voltage regulator, e.g. a stabilized series voltage regulator.
The invention is based on the recognition of the fact that, as long as the operation voltage is kept above a defined minimum voltage, some integrated circuit signal processing parts of a digital hearing aid, like e.g. digital filters, are less sensitive to variations in the operation voltage in the sense that such variations would not result in any significant change of performance.
It is therefore the object of the invention to provide a digital hearing aid having a longer active battery life and a reduced power consumption.
The invention, in a first aspect, provides a hearing aid comprising a microphone, an output transducer, a digital signal processor interconnected between the microphone and the output transducer, a power source and a power voltage regulator, wherein said digital signal processor comprises a first integrated circuit signal processing part and a second integrated circuit signal processing part, wherein said first signal processing part is adapted for operating at a power supply voltage varying within a range above a predefined minimum voltage without significant change of performance, and wherein said power voltage regulator comprises a switched step-down voltage converter connected between said power source and said first signal processing part and adapted for providing a power supply voltage varying above said predefined minimum voltage.
The invention, in a second aspect, provides a hearing aid comprising a microphone, an output transducer, a digital signal processor interconnected between the microphone and the output transducer and a hearing aid battery for the supply of operation volt-age for said digital signal processor, wherein said digital signal processor comprises a first integrated circuit signal processing part and a second integrated circuit signal processing part, wherein said first signal processing part is adapted for operating at a power supply voltage varying within a range below a nominal voltage of said battery without significant change of performance, and wherein said power voltage regulator comprises a switched step-down voltage converter connected between said battery and said first signal processing part and adapted for providing a power supply voltage varying below said nominal voltage of said battery.
The invention, in a third aspect, provides a digital hearing aid comprising a microphone, an output transducer, a digital signal processor interconnected between the microphone and the output transducer and including one or more integrated circuit signal processing parts, and a power source including a standard hearing aid battery for the supply of operation voltage for each of said signal processing parts, wherein at least one of said integrated circuit signal processing parts is designed to operate at a reduced operation voltage substantially below a nominal voltage of said battery and wherein a switched step-down voltage converter is connected between the power source and said at least one signal processing part for lowering the battery voltage to provide said reduced operation voltage.
By lowering of the operation voltage requirement for parts of the integrated signal processing circuits the total current drain and power consumption of the hearing aid is reduced. In particular, this brings substantial benefits in terms of power consumption in hearing aids, where the digital signal processing is operated by large hardware programmed programs, generally likely to otherwise be associated with significant power consumption.
Preferably, the digital signal processing parts required to operate at reduced and varying operation voltages will be implemented in MOS or CMOS technology using transistors having a low operating voltage, e.g. a low threshold or pinch-off voltage, rather than bipolar processing circuits as normally used in hearing aids. Typically, such signal processing parts will comprise circuits that are not stressed with respect to processing speed or output power demand, such as digital filter circuits, whereas more stressed circuits such as an output D/D converter or output amplifier may still be supplied with a higher operation voltage.
By suitable design of such signal processing blocks, which are stressed in processing speed, involving e.g. a split-up in more parallel or serial processing blocks, the requirements to processing speed and consequently the operation voltage requirement may be lowered even for such circuit blocks.
Thus, in a preferred embodiment of the hearing aid according to the invention said first signal processing part comprises parallel signal processing blocks each operating at said varying power supply voltage. The reduced operation voltage for the signal processing parts in question would preferably be equal to or below 0.8 V, e.g. in a voltage range of half the nominal battery voltage, such as 0.7 down to 0.4 V, or preferably 0.65 down to 0.5 V.
In a preferred embodiment the switched step-down voltage converter providing the reduced operation voltage or voltages would be a capacitive charge pump converter, which may advantageously be designed to deliver two or more output voltages. However, alternatively also a switched inductor type converter could be envisaged.